1. Field of the Invention
The present invention relates to a full-color organic electroluminescent device (abbreviated as FOELD), and more particularly, relates to an full-color organic electroluminescent device (FOELD) which has a fundamental structure composed of two electrodes with a light emitting layer interposed therebetween to emit a light with the wave length approximately between 380.about.400 nm close to ultra violet ray region and to a process for this fabrication.
2. Description of the Prior Art
A FOEL device is a direct current driven light emitting device with a fundamental structure composed of two electrodes with a light emitting layer interposed therebetween. When a bias voltage is applied on the OEL device, positive holes and electrons are emitted from the anode and the cathode respectively. Then after having passed over their corresponding energy barriers, they meet with at the light emitting layer to form excitons. A light is emitted therefrom in radiation mode by degenerating those excited excitons to the ground state. The color of light exhibited by such a device is closely related to the light emitting material employed. If a light emitting material possesses a relatively larger energy gap, the device formed will emit a blue light or an ultra-violet light, while the device emits a red light if the light emitting material possesses a smaller energy gap. For a light emitting device to deliver light, at least one of the electrodes should be transparent, made of a thin film material such as Indium Tin Oxide (ITO).
It is advantageous that an OEL is a low energy consuming device, able to generate light of various color with the brightness as high as above 6000 cd/m.sup.2, with a broad viewing angle as broad as 160.degree., and with the response time as rapid as only several .mu.s, which is 100 times quicker than that of a LCD. Moreover, its manufacturing process is much more simple and easier than that of a LCD, and the total thickness of an OELD is only about 2 mm. Its major weak point, short life, of initial stage, has been overcome and improved to be as durable as to reach about 20,000 hours, so that it has a very strong competitive potential versus LCD in the present market.
It is a dream for the manufacturers to make a full-color OELD capable of emitting light without a back light source, with the features of energy saving, thin in dimension, and easy manufacturing, and highly commercially competitive versus LCD in the market.
There are three methods for manufacturing a full-color organic electroluminescent device (FOELD):
1. Using a white light organic electroluminescence as a basic element to make the organic EL emit white light. R, G, B original light are produced by placing a light filter on its surface. PA1 2. Using a blue light organic electroluminescence as a basic element, and adding a wave-length conversion layer on its surface to convert a part of blue light into red light, and another part of blue light into green light, R, G, B three primary light are thus produced, as disclosed by Eastman Kodak Co. in U.S. Pat. No. 5,294,870, 1994. PA1 3. Directly using an organic electroluminescence of R, G, B three lights as a basic element to form a full-color device as disclosed in patent No. 318,966 UROC and No. 294,842 ROC, such a structure is theoretically most ideal. However, at present, except the blue light organic ELD which can emit pure blue light, the red and the green light organic ELD can not produce pure green and pure red lights. It is rather complicated in manufacturing processes to put organic ELD of various color together, this is the reason why a blue light OEL element is utilized as the fundamental structure for producing R, G, B three light, as described in above method 2.
The method described in method 1 has a low brightness due to a light filter is placed on the surface of OEL, accordingly, its light generating efficiency is rather low. On the other hand, in the second method described in method 2, the mixed hues of blue-red and blue-green are produced first, a light filter layer is inevitably required to filter out the blue light such that the brightness is reduced, the production cost increased, and the manufacturing process is more complicated. As for the third method, this is the worst among three, by comparing the complexibility in manufacturing process, high cost and the non-homogeneity of brightness in the device.